Don't do it. It is faster to take the tank out than it is to properly prep, cut, and metal finish an access door. Lowering the rear is easy and only requires the removal of a few bolts. You just have to let it sag. You don't need any special tools other than a socket set and maybe a few wrenches. It isn't that hard but for some reason everyone thinks it is so much harder.
If you don't have the tools to do it right, i would get them. Or have to towed to a shop to do it. Its a tight fit to get a good cut, so the best and safest way to prevent injury to the tank would be to do it with the tank out. Thats just my opinion.
